In order to elucidate factors affecting seasonal and interannual variability in crude fat content (Fc) and condition factor (Cf) of the Pacific stock of chub mackerel Scomber japonicus, we examined Fc and Cf in relation to fork length (Fl), capture date (Cd, number of days elapsed from January 1st in each year) and year. We measured Fc, Fl and body weight of 2,085 individuals of chub mackerel (21–42 cm Fl) captured by purse seine fleets from waters off northeastern Japan during 2012–2017. Fc and Cf were high from September to January, and slightly decreased in February. During March–August, when sample size was small, Fc was lower than in February, and Cf level was close to that in February. Non-linear relationships were detected between Cf and Fc, between Fl and Fc, and between Fl and Cf. The best GAM model in terms of AIC, with Fc or Cf as a response variable, and year (categorical variable), Cd and Fl (spline functions regardless of their independent or simultaneous usages) as explanatory variables, effects of year were statistically significant. Potential causes of strong positive effects of 2016 and 2017 on Fc were discussed.